Overall...Yeah, I'd say they very much are the future, and no doubt have somewhat put a hurt on leather sheaths, BUT...I'm a firm believer that there's still plenty of room for leather to exist.
Kydex and Tek-locks do offer multiple carry positions, good retention, quick access, etc. But...it also scratches the hell outta knives, and can make alot of noise when yer huntin' heavy brush areas. Also I've taken a dive or two, and walked into doorways that have cracked and broken mini Tek-Locks.
Compare that to leather that doesn't hurt a knife at all (unless it gets wet and isn't taken care of) and also very quiet, and flexible with various sorts of impact. Also I've found that it's got much less of a questionable nature when ya' carry a fixed blade as an EDC around the public at large. (Sheeple..sheesh, go figure, eh?)
Also, most Tek-Lock sheathes I've seen (mainly talkin' fixed blades here) just ride way the hell too high for me. It makes it hard as hell to access alot of my knives in a normal, forward grip. Thusly, they wind up being carried in a reverse grip, in order to prevent my looking and feeling like an inbred, herniated gorilla trying to reach a hard to scratch spot. Well...some folks would say I resemble forementioned primate anyways, but that's for a different discussion...
Of course that's mainly a design thing, but...it still seems to be the trend in factory knives. Cripe, drop the knife down a little, I'll wear a longer shirt, and also do without the rib tickling.
Point being that the Tek-Locks and kydex do have lots of great bennies, but a quality made leather sheath will NEVER be anything to turn yer nose up at.